1. Field of the Invention
The object of the invention is a sheet, especially an electric sheet for manufacturing laminated iron cores for generators, motors, transformers, etc. and covered on one or both sides with a thin layer consisting of an adhesive applied as a liquid to the sheet and then dried and of a filler.
2. Discussion of the Prior Art
In manufacturing laminated iron cores, the sheets are stacked one on top of another, separated by a coating to decrease eddy-current losses, and are usually already coated on one or both sides. The coating can be for example a lacquer, specifically a hardened plastic-resin lacquer, or an inorganic phosphate coating, something called a bonder layer. Since it is basically attempted in this method of manufacturing laminated iron cores to provide the laminated package with as high a stacking factor (volume of iron per volume of package) as possible, the coating between the sheets must be as thin as possible. The stacked sheets are then fastened together, by riveting or welding for example, to secure the package against the mechanical stress that results in practice from the dynamic oscillations produced by alternating magnetic fields.
To secure the laminated package against mechanical stress without, however, riveting or welding the stacked sheets together, coating the sheets ahead of time on one or both sides with adhesive, specifically with a hardening adhesive applied in a liquid film to the surfaces to be bonded together, is known. The adhesive can be preliminarily dried at low temperatures (room temperature to about 150.degree. C.), in which state it is not adhesive. The preliminarily dried adhesive can be subjected to higher temperatures to resoften it (plastomeric state) and then duroplastically hardened under pressure at the higher temperature, ranging from 130.degree. to 250.degree. C. Subsequent to cooling to room temperature while maintaining the pressure, the adhesive will develop full adhesion.
Since the preliminarily dried layer of adhesive is non-adhesive at room temperature, the material, which is in the form of a strip, can be coated and preliminarily dried before it is wound into coils and supplied for final manufacture of the iron cores, at which time the particular sheets selected for the laminated package are stamped out of the strip of material and collected into stacks. Subsequent compression of the sheets or package under high pressure and simultaneous heating to hardening temperature following the softening and hardening of the adhesive applied to one or both sides of the sheet leads to adhesion. The hardening temperature is usually higher than the temperature at which the adhesive, applied as a liquid, has been dried. Experience has shown that hardening time is also a determining factor. Subsequent to hardening, the package is cooled to room temperature while the pressure is maintained.
When the sheets are coated exclusively with adhesive, tests have demonstrated a drawback in that it is impossible to always prevent metallic contact between the sheets in the laminated package because the coating gets softer as temperature increases during the compression and simultaneous melting and hardening of the adhesive and is accordingly forced away from any elevated points on the surface of the sheet. In the adhering state, this leads not only to increased eddy-current losses but also deleteriously affects the reliability with which the adhesive adheres. Surface irregularities that can force the adhesive away or out include deviations in shape or thickness, warping, surface defects like pimples and scratches, the stamping burr, and even the roughness of the sheet itself. Problems have developed in particular in fastening larger packages together, in assembling cores for transformers or stator packages for generators, for example.
Another drawback to sheets that are coated strictly with the adhesive, preliminarily dried, and assembled into a laminated package is that the adhesive, which escapes at the side and dries while it is being compressed, must be subsequently removed mechanically at considerable expense. Although this can be taken care of by grinding, the resulting metal burr must then be removed in an additional step. Attempts to prevent the adhesive from escaping by applying the pressure instantaneously have not been successful because the quality of the adhesive bond depends also to a considerable extent on the length of time the pressure is applied. Furthermore, final-annealed electric sheets are sensitive to impacts and jolts, the more so the higher their quality. The instantaneous application of pressure when compressing the sheets can, accordingly, not be utilized to prevent the adhesive from escaping out of the package.
Improving the magnetic properties of laminated iron cores made out of single sheets for transformers, motors, generators, and similar devices by coating both sides not with adhesive alone but with a mixture of adhesive and filler that is subsequently dried is already known from U.S. Pat. No. 3,029,403. The filler consists of a number solid, spherical, electrically non-conductive particles. The particles are preferably spherical glass beads with a uniform diameter of about 25 .mu.m. The sheets coated with this mixture are stacked into a laminated package and fastened together by compressing the sheets or package under pressure and simultaneous heating.
As tests of this known method demonstrate, however, the aforesaid drawbacks and problems deriving from warping, surface defects and even the roughness of the surface of the sheet cannot be overcome with this process. It has been demonstrated that the sheets in the finished laminated packages will still come into metallic contact. Furthermore, the embedding of spherical particles in the adhesive does not prevent it from escaping at the sides while the sheets are being compressed subject to pressure. Also, tests have demonstrated that it is especially difficult to obtain and in particular to preserve a homogeneous mixture of the adhesive and spherical particles not only while the coated and preliminarily dried sheets are being manufactured, but, even more so, during the subsequent fastening together of the sheets into a package. The spherical, freely moving particles can wander through the layer of adhesive and avoid the irregularities, e.g. the peaks of roughess for example, in the surface of the sheet not only in the liquid phase of the adhesive while it is being applied to the sheets, but also in the thermoplastic phase during compression and adhesion.
This essential drawback of the known method has only been reliably eliminated by keeping the diameter of the spherical solid particles essentially longer than any conceivable surface irregularities. This, however, results in an unsatisfactory stacking factor and a large and undesirable interval between the individual sheets in the laminated package.
It has in general been demonstrated that it is impossible to avoid double coating in manufacturing laminated iron cores by fastening together stacked sheets of an especially large surface area--supporting-magnet sheets for longitudinal stator drive mechanisms in magnetic levitation, stator and rotar end sheets for large electrical machines, or yoke sheets for large-scale transformers, for example--if the eddy-current losses are to be reduced to the necessary level. In double coating, the sheets are first coated on one or both sides with a conventional electric-sheet coating like a hardened artificial-resin lacquer or bonder layer. The adhesive is then applied in liquid form to this layer and dried. Of potential double coatings, only hardened artificial-resin lacquer in combination with an adhesive applied thereto has led to satisfactory results with respect to the desired decrease in eddy-current losses. Aside from the essentially higher costs of double coating, the essential drawback of this process is that it can generally only be carried out in two separate stages. This is also true, for example, when one side of the strip of material or sheet is provided only with the adhesive and the other side with a conventional electric-sheet coating.
Another potential drawback of double coating derives from the unsatisfactory adhesion of the adhesive to the conventional electric-sheet coating. The result is that qualitative trade-offs in the adhesion of the sheets to each other must absolutely be taken into account. Again, these drawbacks can only be compensated for to some extent by adding even more expensive adhesion promoters.
The object of the present invention is to provide a sheet that is coated on one or both sides with a preliminarily dried adhesive and that can be employed to manufacture laminated iron cores with no metallic contact between the sheets even when there are surface irregularities without reducing the requisite reliability of adhesion and without any of the adhesive escaping laterally out of the package while the sheets are being compressed under pressure.